Apparatus and method for supplying material to a substrate

ABSTRACT

An apparatus and method are described for supplying material to a substrate (109). The apparatus includes a member (3) having a surface with a plurality of features (8) which locate, in use, menisci of a liquid (1) supplied to the member. An actuator (4) induces mechanical vibrations within the liquid located by the features to cause liquid droplets (7) to be sprayed. Liquid (1) is supplied to the member and electrical charge is supplied to the member and electrical charge is supplied to the liquid by, for example, an electrode (14). Electrical charge or potential is also supplied to the substrate (109) so that the droplets are directed towards the substrate to deposit material thereon.

This invention relates to an apparatus and method for the supply ofliquid droplets and/or solids that are at least initially carried byliquid droplets, the droplets having an electrical charge. Moreparticularly, the invention relates to the supply of liquids and/orsolids into a gaseous environment.

The invention further relates to an apparatus and method for supplyingliquid and/or solids to a substrate having upon or below its surface anelectrical charge or potential, including cases where that electricalcharge or potential is in the form of a spatial pattern within thesurface area presented by the substrate to the droplets or solids.

In this specification we refer as `liquids` to the following: pureliquids, mixtures of pure liquids, solutions of solids and suspensionsof particulate solids in any of the above. The term `liquid droplet` issimilarly to be understood to include droplets of mixtures, solutionsand suspensions as well as of pure liquids. In the case of solutionswhere we wish to refer specifically to the solvent rather than thesolute, and in the case of suspensions where we wish to refer to thesuspending liquid rather than the suspensate, we refer to the `carrierliquid`.

In this specification we also refer to liquid `conductivity`. By this wemean the ability to conduct an electrical current through the liquidfrom electrodes at differing electrical potentials immersed in theliquid. This includes the motion of charged solute or suspensate species(including solid particles) within the carrier liquid, which currentwould not occur in the absence of such species.

It is known to deposit liquids and/or solids materials on to substrates,the liquids and/or solids materials being carried to those substrates inthe form of droplets of liquid (as herein defined) or of powderedsolids. Applications include: the coating of moving sheets of substratematerial, for example, to manufacture products such as adhesive tapes;the deposition of protective layers upon functional substrates otherwisevulnerable to their environment; and to confer specific properties ormodify the properties of the substrate material, for example, coatingsthat control the release of a drug from a drug-containing matrix, theapplication of toner material in electrographic process, etc.

In some of these arts, for example in the electrographic andelectrophotographic imaging arts, it is desired that the deposition ofsuch airborne droplets (or powder solids in the case of evaporation ofthe carrier liquid before arrival at the substrate) on a substrate isresponsive to a pattern of electrical charge or potential on or belowthe surface of that substrate. To enable this, it is generally requiredto provide the droplets with an electrical charge. For faithfuldeposition according to the pattern of electrical charge or potential ofthe substrate it is also generally required that the droplet inertiashould not be too large (in relation to the electrostatic forces exertedon the droplets by the charge or potential pattern of the substrate), sothat the motion of the charged droplets towards the substrate isresponsive to the electrostatic forces between the substrate and thedroplets and is not primarily governed by the momentum with which thedroplets (or powder solids) enter the region proximate to the substrate.(This is also desirable, though less critical, in the case of depositionupon substrates whose charge or potential is uniform over the surfacearea of the substrate presented to the droplets.) In this way so called`imagewise development` known in the electrographic imaging and printingarts that renders visible a pre-written pattern of electrical charge bydroplets containing opaque solids particles or dyes has been achieved.Particular examples are described in U.S. Pat. Nos. 3,005,726 (Olson);2,690,394 (Carlson); 3,532,495 (Simm); 3,795,443 (Heine-Geldern). Inother arts, it may not be an object that a visible mark is made by thepattern of solids remaining after evaporation of the carrier liquid.

Hitherto, however, whilst known spray deposition methods are capable ofdepositing droplets according to a pattern of charge or potential,various drawbacks have limited their adoption for applying toners in theelectrographic imaging and printing arts and for applying liquids orsolids upon substrates in other deposition arts.

In many applications a high density of droplets in the surrounding gas(usually air) is often desired so that the process can be rapid. Thefreedom to use liquids of a wide range of electrical conductivity isalso desired, to give greatest applicability. It is generally desiredfor the apparatus to be simple, compact, and low in cost to allowcommercial use in a wide range of applications. Finally, especially inelectrographic imaging and printing applications, it is desirable toproduce small droplets (typically less than 40 μm in diameter) in orderthat their arrival on the substrate surface can accord with the finedetail of the charge image. In such applications the electrical chargeupon the substrate is often (although not always) somewhat limited, afinite quantity of charge having been deposited on insulating substratesby sources such as corotrons. It is correspondingly desirable for thedroplets to have a well-controlled ratio of electrical charge to mass.Separate control over droplet size and charge level is thereforedesirable.

Existing methods of aerosol production, including electrostaticatomisation, continuous ink jet (CIJ), ultrasonic atomisation andpressurised spray nozzles are unsatisfactory in various ways for suchapplications.

In the electrostatic spray deposition art the droplet formation andcharging processes are inextricably linked. It is therefore difficult orimpossible separately to control the charge and the size or inertialbehaviour of droplets so generated. Even though large electrostaticfields are employed to generate the droplets (generally by electrodes athigh electrical potential in front of the liquid meniscus), the initialinertia of the droplets so produced is of such magnitude that theyescape from these very high electrostatic fields with considerableretained inertia. This makes the kinetic response of such droplets tothe generally weaker electrostatic fields of charge patterns formed onsubstrates rather poor. Consequently electrostatic spray deposition, tothe knowledge of the inventors, has hitherto been limited to depositiononto substrates having little or no spatial variation in the pattern ofcharge or potential within the surface area presented by the substrateto the droplets. Further, electrostatic droplet generation is rathersensitive to the electrical conductivity of the carrier liquid, solimiting its practical utility. One successful application ofelectrostatic spray deposition has been spray painting, but no practicalgeometries to produce high densities of droplets for rapid `imagewise`deposition (as described above) in compact equipment is known to theinventors and electrostatic spray deposition has not found generalapplication in higher-resolution deposition, such as electrographicprinting.

Continuous ink jet (CIJ) devices issue a jet of pressurised liquid fromeach of many orifices, which jets break up into droplets under theinfluence of a vibration source. Droplet separation generally occurs inthe vicinity of an `induction electrode`. A separate such inductionelectrode is positioned in front of each orifice and induces charge toflow into each jet and thence into each forming droplet. CIJ devicestherefore separate the droplet formation and charging processes, givinggreater control. However, they employ individual electrostatic controlof the charging of each separate jet. To the knowledge of the inventors,such devices designed to deposit droplets on substrates according to thedroplet charge produce relatively large droplets (typically 60-100 μmdiameter) at relatively low frequencies (typically less than 150 kHzdroplet generation rate per orifice). The inertia of each chargedindividual droplet is again sufficient reliably to escape theelectrostatic attraction of the `induction electrode`. On entering theregion proximate to a substrate (having upon or below its surface apattern of electrical potential or charge), it is again difficult toarrange that the droplet motion towards the substrate is primarilygoverned by the electrostatic forces exerted on the droplets by theelectrostatic field pattern presented by the substrate. Ultimately ofcourse, the viscous drag of the air can slow such droplets downsufficiently that they can respond to such electrostatic field patterns.However, this requires large distances between droplet generation andsubstrate, so that compact apparatus is not provided; further the largedroplet inertia makes their response slow. It is also found thatgravitational settling of the relatively massive droplets, rather thanpurely `imagewise development`, can occur. Still further, on arrival atthe substrate a large `mark`, corresponding to the large droplet size,is produced. CIJ techniques known to the inventors therefore do notenable imagewise development in compact apparatus and in particular donot enable deposition according to charge or potential patterns of highspatial frequency.

Ultrasonic atomisation from unconstrained liquid surfaces (as describedfor example by Rozenberg in Physical Principles of UltrasonicTechnology, published by Plenum) may be integrated with electrodes toimpress charge upon droplets as or after they are generated (see forexample U.S. Pat. No. 2,690,394, Carlson). These methods create a highinitial density of droplets and can produce small droplets. Howevertheir wide initial droplet size distribution generally require means toselect the desired size fraction, which results in a low density ofdroplets at the final substrate and in bulky equipment. These ultrasonicatomisation methods generally produce sprays in the form of anear-stationary `mist` above the liquid surface (see for example U.S.Pat. No. 3,795,443, Heine-Geldern), so that droplet charging by means ofan induction electrode such as that described for continuous ink jetprinting above is unsatisfactory--insufficient numbers of the dropletsthen have sufficient inertia to escape the electrostatic field of theinduction electrode for effective utilisation of the liquid. Recovery ofsuch `wasted` liquid from the electrode is also generally required.

Pressurised nozzle systems also produce wide droplet size ranges andexcessive droplet velocities.

As a result of these problems, particularly but not exclusively in theelectrographic imaging and printing arts, the aerosol method fordepositing liquids and/or solids has not been extensively adopted.

An object of the present invention is to overcome various problemsassociated with the prior art charged-droplet supply apparatus.

A further object is to provide apparatus capable to supply, in the formof charged droplets and to substrates having upon or below their surfacean electrical charge or potential, liquids and/or solids whosedeposition upon said substrate is responsive to said substrate charge orpotential. The charge or potential on the substrate may be disposed in apattern.

According to a first aspect of the present invention there is providedapparatus for supplying material to a substrate, said apparatuscomprising:

a member having a surface, a plurality of features at said surface forlocating at said surface, in use, menisci of a liquid supplied to saidmember;

liquid supply means for supplying liquid to the member;

means for supplying electrical charge to the liquid;

an actuator for inducing mechanical vibrations within the liquid locatedby said features to cause charged liquid droplets to be sprayed fromsaid member; and

means for providing electrical charge or potential to the substrate,whereby said charged droplets are directed towards said substrate todeposit said material thereon.

In the context of the present specification when reference is made tosupplying electrical charge or potential "to" the substrate it is to beunderstood that this means either directly to the surface of thesubstrate or above or below it.

The invention also includes a method of supplying material to asubstrate, said method comprising:

supplying liquid to a member having a surface, said surface having aplurality of features locating menisci of said liquid at said surface;

inducing mechanical vibrations within the liquid located by saidfeatures and causing liquid droplets to be sprayed from said member;

supplying electrical charge to the liquid; and

providing electrical charge or potential to the substrate, whereby saiddroplets are directed towards said substrate to deposit said materialthereon.

The supply of liquid to the member may be "on-demand", in other wordsreplenishing, so that liquid is supplied to match the spray of dropletsfrom the member.

The features may be in the form of orifices capable of allowing liquids(as herein defined) to pass through them. Conveniently, though notnecessarily, the member will take the form of a perforate plate ormembrane, the orifices or, equivalently, perforations extending betweentwo substantially parallel faces of such a plate or membrane. Theorifices may be permanently open or closable when liquid is not passingthrough them (for example if the member is a rubber or similarmembrane). The liquid will typically be brought to one face of thatplate or membrane.

For ease of reference only, the present invention will be describedhereinafter by reference only to such perforate plates or membranes,which forms in the experience of the inventors convey greatestadvantage. Application to other forms of member incorporating orificesor other features, eg. surface relief formations, such as thosedescribed in EP-A-0615470, is to be understood.

The means for supplying charge or potential to the liquid may supplyfree charge conductively through the liquid before or as the dropletsare generated; alternatively the means for supplying charge may supplyfree charge to the droplets once formed; both as further discussedbelow.

Further alternatively, in the case where the liquid itself containscharged species, the `on-demand` or replenishing supply of liquid mayitself be used to bring further charge to liquid adjacent to theperforate region of the plate and thence to the droplets.

In use the perforate region of the plate is contacted on one face(hereinafter termed the `rear` face) by bulk liquid and is contacted onthe opposing face (the `front` face) by a gaseous medium, usually air.However, hereinafter wherever the term air is used gases generally areto be understood as included.

Vibration of the element or plate by the actuator, particularly atultrasonic frequencies, induces liquid to pass through the orifices andto emerge from the front face as individual droplets moving through theair away from the plate or element. In particular, the simultaneousejection of multiple droplets creates a cooperative droplet transporteffect, particularly in the region immediately in front of the perforateplate (and in which region an optional `induction electrode` may besituated), that enables droplets to be charged by and to `escape` fromthe apparatus, and yet for those droplets to present low inertia inrelation to the electrostatic forces exerted upon them by a substratehaving upon or below its surface a pattern of electrical charge orpotential.

This desirable effect is particularly marked in the case of smalldroplets (of diameter less than, say, 40 μm and of more typical diameter5 μm-20 μm) ejected with initial velocities in the range 5 to 15 metersper second at an initial spacing (in the plane transverse to thedirection of ejection) typically in the range of 200-500 μm.

The mechanisms involved in the operation of the apparatus and method ofthe invention are believed to be as follows:

Consider first droplet ejection. Such typical small droplets, if ejectedfrom single orifices or perforations, are rapidly decelerated by theair, coming to near-stationary motion very close to the ejectingperforation (generally within a few millimeters). For example, use ofinduction charge electrodes, as in conventional CIJ apparatus, with suchsmall droplets is not expected to allow droplets reliably to escape fromthe strong electrostatic fields of the induction electrode.

However, it has been found that the use of multiple closely-spacedorifices or perforations, all ejecting droplets simultaneously, producesa droplet stream upon which the effects of viscous deceleration by airare greatly reduced. It is believed by the inventors that the viscousdrag can now act effectively only upon the outer surface of the overalldroplet stream, not upon individual droplets, and that such a dropletstream has sufficient initial momentum to entrain air flow with thedroplet stream. In this way the initial viscous drag experienced by thedroplets is reduced and so, despite their low size, they can betransported away from the apparatus. Indeed, in the case of charging ofsuch droplets by means of an induction electrode, the great majority ofsuch droplets in such a droplet stream can now escape past the inductionelectrode whereas, if the droplets were ejected from a single orifice orperforation (but otherwise under the same conditions), many would becaptured by the induction electrode.

Consider next droplet deposition upon substrates having upon or belowtheir surface a pattern of charge or potential. The charged dropletswithin the ejected droplet stream produced by the claimed apparatusincorporate air within the stream, initially slowly. If charged with asingle sign of charge, which is generally desirable, they also repeleach other electrostatically. Both effects cause the droplet stream tospread sideways (i.e. substantially perpendicular to their direction oftravel), and thereby to encounter and incorporate more and more airwithin the droplet stream. The droplets thereby (and aided by theirsmall mass) rapidly decelerate, having greatly reduced velocities ashort distance away from the perforate plate (between 5 and 15centimeters for typical embodiments) in the form of a dense `cloud` ofdroplets. In this condition the low inertia of the droplet cloud allowsdroplet migration to the substrate that is highly responsive to theelectrostatic field pattern that the substrate presents to the droplets.This enables faithful deposition according to that pattern.

Charge may, for example, be impressed upon the ejected droplets ofconductive liquids brought to the perforate plate by an imposed electricfield in the airspace (in general taken to mean `gas space` in theapplication) at or closely in front of or behind the perforate plate,together with electrical contact of the water to a source of freecharge.

Free charge may also be brought to the ejected droplets by exposing themto an ion source such as a corotron or an `electrogasdynamic` sourcesuch as that described in U.S. Pat. No. 3,606,531. Such methods areindependent of the conductivity of the droplet itself and so allowcharging of electrically insulating liquid droplets.

As a third example, electrical charge may be brought by a replenishingsupply of liquid that replaces liquid ejected as droplets. Examplesinclude both conducting liquids such as aqueous solutions andsuspensions, and insulating liquids carrying separated charge specieswithin them. An example of the latter is `liquid toner` as known fromand used in the electrographic imaging and printing and printing arts.Such liquids which generally comprise an insulating carrier liquid, suchas an iso-paraffin, carrying solid pigment particles (`toner particles`)in suspension and optional further materials such as so-called `chargecontrol agents`. The general electrical configuration of such liquids isthat in which the toner particles acquire a net charge relative to thecarrier liquid while the overall liquid remains electrically neutral.

Finally, in the case of insulating carrier liquids, the droplets may betriboelectrically charged by the passage of the liquid through theperforations of the plate or relative to other surface features thatlocate the liquid menisci.

The present invention thereby combines the virtues of providing chargeddroplets with sufficiently low inertia and small droplet size that theydeposit according to the pattern of electrostatic field presented by adeposition substrate, including the case where that pattern has highspatial resolution, all from compact simple apparatus.

In addition: (i) the apparatus is not strongly sensitive to theconductivity of the liquid, and can operate with liquids of a wide rangeof surface tensions and a range of viscosities at least comparable toother techniques, (ii) in some implementations the size of theperforations has a marked influence on the size of the emitted drople;fabrication of plates with uniform hole size therefore contributes toformation of a droplet stream with the desired narrow size distributionand by this means allows separate control over droplet size and charge,(iii) unlike prior art ultrasonic droplet generation devices having anunconstrained free surface, the perforate structure of the plate allowsdroplet ejection to occur with `droplet-emitting` points that may becontrolled separately from droplet size. Inter-droplet collisions canthereby be suppressed, better maintaining a relatively narrow sizedistribution as the droplets move through the gaseous medium.Sufficiently high density can however still be maintained for rapiddeposition upon substrates, and in particular for rapid imagewisedevelopment of charge images in the electrographic arts.

In particular the inventors find that high conductivity liquids such asaqueous liquids, including aqueous liquid toners, can be satisfactorilyejected as charged droplets by such apparatus, and that these cansubsequently be deposited upon substrates according to a pattern ofelectrical charge or potential upon on below the surface of thesubstrate.

The means for providing a pattern of electrical charge or potential uponor below the surface of the substrate upon which the liquids and/orsolids are to be deposited may be any of the conventional means known inthe electrostatic spraying of electrographic imaging and printing arts.Examples include: (i) the connection of conducting substrates to asource of electrical potential; (ii) the deposition of conducting layersupon electrically insulating substrates in the pattern corresponding towhich liquid and/or solids deposition is desired and then the connectionof said conducting layers to a source of electrical potential orapplying to said layers an electrical charge; and (iii) the use ofso-called `corotrons`, `ionographic heads`, `electrogasdynamic` iongenerators or radioactive decay sources to supply free ions in the airthat deposit on the surface of said substrate. Where these are incapabledirectly of writing a pattern of charge but deposit only unpatternedcharge, they may be used in conjunction with substrates made ofphotoconductive or photoresistive material such that pre-charging orpost-charging exposure of the surface of the substrate to a lightpattern results in the deposited charge also forming a correspondingpattern.

Forms of the perforate plate droplet generation elements of theapparatus described herein that are believed suitable include thosedisclosed in: GB-B-2,240,494; GB-B-2,263,076; GB-A-2,272,389;EP-A-0,655,256; WO-A-92/11050; EP-A-0,480,615; EP-A-0,516,565;WO-A-93/10910; WO-A-95/15822; WO-A-94/22592; U.S. Pat. No. 4,465,234;U.S. Pat. No. 4,533,082; U.S. Pat. No. 4,605,167; WO-A-90/12691; U.S.Pat. No. 4,796,807; WO-A-90/01977; U.S. Pat. No. 5,164,740; U.S. Pat.No. 5,299,739; the entire content of which disclosures is herebyincorporated by reference.

The presently preferred form of perforate-plate droplet generator foruse with the present invention known to the inventors is described inWO-A-95/15822. This device has the capability to deliver relativelysmall droplets from relatively large perforations and allows delivery ofsuspensions of solids particles within carrier liquids as very smalldiameter droplets (for example, less than 10 μm diameter) without thosesolids inducing blockage of the perforations. This is beneficial inapplications such as image-wise delivery of toner suspensions inelectrophotographic imaging and printing applications. This also allowsthe use of plates or membranes with hole sizes that are relatively easyto fabricate and thus relatively inexpensive.

Preferred embodiments of the invention will now be further described byway of example only and with reference to the accompanying drawings, inwhich:

FIGS. 1a, 1b: show sectional and plan views of a droplet dispensationand charging apparatus

FIG. 1c: shows a partial enlargement of FIG. 1a, illustrating thecircumscribing of the menisci of liquid sprayed from the apparatus byorifices in a perforate plate or membrane

FIG. 1d: shows an example of a means for providing electrical charge orpotential to the substrate shown in FIG. 1a

FIG. 2a: is a sectional view of a second droplet dispensation andcharging apparatus

FIG. 2b: is an electrical circuit suitable for exciting vibration in theapparatus according to any of FIGS. 1 to 13

FIG. 3: is a sectional view of a droplet dispensation and chargingapparatus with an induction electrode

FIG. 4: is a sectional view of a second droplet dispensation andcharging apparatus with an induction electrode

FIG. 5: is a schematic section of a droplet dispensation and chargingapparatus suitable for use with liquids carrying charge species but thatare otherwise are non-conducting

FIG. 6: is a schematic section of a second droplet dispensation andcharging apparatus suitable for use with liquids carrying charge speciesbut that otherwise are non-conducting

FIG. 7: is a schematic section of a third droplet dispensation andcharging apparatus suitable for use with liquids carrying charge speciesbut that are otherwise non-conducting

FIG. 8: is a schematic section of a droplet dispensation and chargingapparatus in which droplet production occurs as a result of vibrationsinduced within the liquid

FIG. 9: is a schematic section of a second droplet dispensation andcharging apparatus in which droplet production occurs as a result ofvibrations induced within the liquid

FIG. 10: is a schematic section of a droplet dispensation apparatus inwhich droplet charging occurs after droplet dispensation

FIG. 11: is a schematic section of a further embodiment of an apparatusaccording to the invention

FIG. 12: shows a further example of a means for providing electricalcharge or potential to the substrate shown in the above figures.

FIGS. 1a to 1c,2a,3 and 4 show embodiments suitable for conductivesupply of free charge to conducting liquid. FIGS. 5 to 8 showembodiments in which the supply of liquid itself supplies further chargeas charged droplets are ejected. In the cases of FIGS. 1 to 8 is showndroplet production by the action of a vibrating perforate plate ormembrane. FIGS. 9 to 10 show similar embodiments to selected forms fromFIGS. 1 to 8 but in which droplet production is effected by inducingvibration directly within the liquid rather than inducing vibration ofthe perforate plate or membrane in order, in turn, to induce vibrationof the liquid.

FIG. 1a shows a first embodiment having a generally circular geometry.In this example, conducting liquid shown at 1 is brought into contactwith at least the perforate region of the rear face 2 of a perforateplate or membrane 3 by a supply means 16 (shown schematically as asyringe body) and in which a circular piezoelectric vibration actuator4, under the influence of an alternating electrical power source 5(supplying an alternating potential V_(act)) causes the plate ormembrane 3 to vibrate in the direction shown by arrow 6. The vibrationresults in liquid being ejected from perforations 8 in the plate ormembrane and for that ejection to be in the form of droplets 7 in thedirection shown by arrow 9 generally towards a substrate 109 AlthoughFIG. 1a shows the droplets being ejected substantially normal to thesurface of the substrate 109, the ejection may be arranged to besubstantially parallel to the substrate surface. In use, theelectrostatic field presented by charge or potential on or below thesurface of the substrate 109 (as further described below) stillultimately directs the motion of the droplets towards the surface of thesubstrate.

The vibration provided by the actuator 4 is coupled directly to plate ormembrane 3, but may alternatively be coupled to the plate or membranevia an intermediate coupling element. The actuator 4 is preferablychosen to operate in the frequency range above 10 kHz. If very smalldroplets, for example 10 μm or smaller diameter, are to be produced theactuator 4 may typically be operated in the range 200 kHz to 5 MHz.

A means 10 to supply free electrical charge to liquid 1 comprises anelectrical supply 11 capable to supply free charge at a potential V_(ch)relative to ground potential (shown at 12) via conductors 13 to anelectrode of a `charge donating assembly` 14 immersed in the liquid.Charge may thence flow to any other conductors in electrical contactwith the liquid and so be donated to droplets emergent from theapparatus. For this reason the assembly of electrical conductors,including the electrode shown in the figure, in electrical contact withliquid 1 is referred to as the `charge donating assembly`. Control ofV_(ch) to differ from the electrical potential of the airspace 15 ashort distance in front of plate or membrane 3 causes the droplets toemerge with an electrical charge, the sign and magnitude of which isresponsive to variation of V_(ch). It is to be noted that the electricalpotential of airspace 15 is in general influenced by the free chargedensity present in that airspace introduced by the charged ejecteddroplets 7.

In the embodiment of FIG. 1 all materials other than the free electricalcharge supply means 10 contacting liquid 1; including perforate plate ormembrane 3, any intermediate vibration coupling means between plate ormembrane 3 and actuator 4 (not shown), and any enclosure for liquid 1(not shown) may be electrical insulating.

FIG. 1b shows a plan view of the piezoelectric actuator 4 and theperforate plate or membrane 3 shown in FIG. 1a. There is shown anelectrode 4a on the upper surface of the acuator. There will, foractuators of this annular circular form, be a similar electrode on theunder surface of actuator 4. (That second electrode is typically aseparate element from plate or membrane 3, and may be electricallyinsulated from it.)

FIG. 1c shows, in enlarged cross-sectional form, droplets 7 of liquid 1emergent from perforations or orifices 8 in the plate or membrane 3showing that the orifices locate, at 17, the menisci of the liquidemerging from the plate or membrane 3 (in this case they circumscribethe menisci at the front of the plate or membrane 3). The separation ofthe orifices may be controlled to limit in-flight coalescence ofdroplets so ejected. Other surface features of member 3, includingsurface relief features of unperforated membranes or plates, may alsoprovide this desired meniscus location effect.

In the understanding of the inventors, free charge flows into the liquidand electrode (and other elements of the charge donating assembly 14)because there is both a finite electrical capacitance between the chargedonating assembly and its surroundings and a difference of electricalpotential with those surroundings. (The "surroundings" may, for theseelectrostatic purposes, be considered to be at an infinite distance fromthe charge donating assembly. The capacitance is influenced by thegeometry of the charge donating assembly). Correspondingly there is adiscontinuity in the component of the electrical displacement D normalto the meniscus surface and a corresponding free surface charge densitys (both as known in the electrostatic arts) across the menisci of theliquid emerging from the perforations. Consequently, as droplets breakoff from the emerging menisci they carry away some charge. As liquid islost from the assembly as droplets, the provision of a continuing supplyof free charge (in this example supplied by electrical supply means 10)allows further electrical free charge to flow into the liquid toreplenish that carried away by the ejected droplets.

FIG. 1d shows one means of providing a uniform area of electrical charge123 on the substrate 109 and alternatively or additionally providing apattern of electrical charge 124. In the example shown, the substrate109 comprises a photoconductive material layer 110 having, on its lowersurface, a conductive electrode layer 112. The photoconductive materiallayer 110, prior to receiving charge, is generally allowed to attain a`dark-adapted` state, as is well known in the electrophotographic arts.The conductive electrode layer 112 is, in this example, held at groundpotential (shown at 113) by a conductor 114.

A corotron ion source 115, comprising a fine wire 116 (elongate in thedirection normal to the figure) raised to a potential V_(w) by anelectrical supply 117, and optional conducting grid elements 118 andscreen elements 119 may also be provided. The potential V_(w) is chosento be sufficiently large that the electrical field in the immediatevicinity of wire 116 is sufficiently large to cause ionisation of theair and thereby to produce a stream of ions that are directed, at leastin part and as shown at 120, towards the surface 121 of the substrate109.

By applying suitable electrical potentials (not shown) to the grid andscreen elements 118 and 119, improved control over the stream of ionsshown at 120, and thereby over the deposition of those ions on to thesurface 121, may be obtained, as is well known in the electrographicarts. In a typical embodiment, the substrate 109 may be moved in thedirection shown at 122 and a uniform deposition of charge shown at 123over an area of surface 121 passing underneath corotron 115 may therebybe provided.

To form a pattern in the deposited charge, photoconductive material 110may, after receiving charge as described above, be illuminated with apattern of illumination causing, through the photo-induced conductivityof layer 110, discharge in regions 124a where layer 110 is illuminatedbut no discharge in regions 124b, where layer 110 is not illuminated.The source of the pattern of illumination may, for example, be ascanning and temporally-modulated illumination source. One such sourceis shown schematically at 125 as a scanning laser source that providesillumination beam 126 that traverses the surface of substrate 109 in adirection normal to the figure.

The apparatus of FIG. 1d is found suitable for use in conjunction withthe apparatus as described with reference to FIGS. 1a to 1c above (andalso further with reference to alternative embodiments as describedbelow) to effect deposition of charged droplets 7 on the surface of thesubstrate 109 according to the pattern of charge represented at 124a and124b. Deposition of charged droplets 7 upon surfaces of insulatingmaterials is similarly found to be effected according to patterns ofelectrical charge or potential formed below such surfaces.

Further, deposition of charged droplets 7 upon surfaces on conductingmaterials is also found to be effected according to the electricalcharge or potential of such materials.

In the example of FIG. 2, the plate or membrane 3 forms part of thecharge donating assembly 14 (and is therefore necessarily electricallyconducting) and thus the electrode of FIG. 1a may be eliminated, and theplate or membrane 3 receives free charge from the source 11 by contact18 and via conductor 13. Plate or membrane 3 therefore donates freecharge to the liquid 1. In this case, if the alternating power source 5is not electrically isolated from ground, then it may be desirable toinsulate electrically (but not mechanically) the plate or membrane 3from the actuator 4 and hence provide electrical insulation from thepower source 5. In the example given of a piezoelectric actuator thismay be achieved by interposing a thin, mechanically stiff, electricallyinsulating layer 19 between actuator 4 and plate or membrane 3.Alternatively or additionally, the alternating power source 5 may beelectrically isolated from ground potential by an isolating transformer20 as shown in FIG. 2b.

In the example of FIG. 3 is shown an induction electrode 25, in front ofthe perforate plate or membrane 3 whose potential or electrical chargelevel is maintained by the electrical supply 11 via conductors 21. Inthis case free charge is supplied at ground potential to the liquid 1(as shown) via electrode responsive to the potential or charge upon theinduction electrode 25. Again the electrode of the charge donatingassembly 14 may be replaced by an electrical connection 18 to aconducting plate or membrane 3 (not shown). Similarly electrical, thoughnot mechanical, isolation of the plate or membrane 3 from the powersource 5 can again be selected as appropriate and as discussed withrespect to FIG. 1.

The inventors understand that, in relation to the example of FIG. 3, theinduction electrode 25 allows the capacitance between the `chargedonating assembly` and its surroundings (and specifically to inductionelectrode 25) to be increased and that, for a given difference inpotential between the liquid and the airspace 15, this allows thediscontinuity in electrical displacement D at the menisci as describedabove to be increased, thereby allowing the droplets to carry away agreater charge. Alternatively, for a given charge on the droplets thepotential difference and therefore typically the magnitude of V_(ch),may be reduced; allowing a simpler or less expensive electrical supply11.

In FIG. 4 is disclosed an alternative electrical arrangement in whichfree charge is supplied to the liquid 1 at potential V_(ch) by theelectrical supply 11, and an induction electrode 25 is connected toelectrical ground potential. This implementation has the advantage, overthat of FIG. 3, of improved electrical safety for apparatus in which the`charge donating assembly` is inaccessible but where the inductionelectrode 25 is accessible to users of the apparatus.

With reference to all geometries in which there are multiple orificessuch that some droplets are ejected in between other droplets from more`central` orifices and the induction electrode it is to be noted thatsatisfactory charging of droplets is surprising and is in markeddistinction to the situation for CIJ induction charging. With particularreference to the circular geometry of FIGS. 3 and 4, charging of thosedroplets at 26 lying towards the centre of the emitted droplet stream issurprising and is in distinction to the situation for CIJ inductioncharging, for which one induction electrode is provided for eachemitting orifice. In the present case of a single induction electrodeand multiple emitting perforations, the droplets at 26 towards thecentre of the stream are surrounded by other charged droplets at 27towards the outside of the stream. These latter are understood partiallyelectrically to `screen` the more central droplets from the influence ofthe induction electrode 25, thereby reducing the discontinuity inelectrical displacement D and hence the surface charge density upon themeniscus of the emerging liquid droplets at the centre of the stream.However, with the present apparatus this is found not to be limiting. Itis believed that this is because inhomogeneous distributions of chargecreate electrostatic pressure gradients acting in the direction toreduce the inhomogeneity and so produce an overall electricallywell-behaved droplet stream. Analogous effects are also believed tooccur with reference to the charging geometries of FIGS. 1 and 2.

In each of the circular-geometry forms shown in FIGS. 2-4 above, withappropriate detailed embodiments, it is found that the simultaneousejection of multiple droplets creates a cooperative droplet transporteffect that enables droplets to be charged by and yet predominantly tobe transported past, induction electrode 25. The electrostatic mutualrepulsion between droplets and air entrainment only subsequently causessubstantial slowdown and spreading of the droplet stream. The result, inthe particular case of the preferred embodiment also further describedwith reference to FIG. 11, is a rather dense cloud of near-stationarydroplets some few centimeters away from the apparatus that is suitablefor deposition on substrates according to a pattern of electrical chargeor potential upon or below the surface of those substrates.

The same cooperative transport effect is also observed with geometriesin which the orifices are arranged in an pattern that is much longer inone direction than another. Linear geometries (where the orifices extendmuch further in one direction than they do in a perpendicular direction)indeed, have particular advantage for deposition of liquids and/orsolids upon substrates moving relative to the apparatus; when, byarranging the long dimension of orifices to lie tranverse to therelative motion between apparatus and substrate, high uniformity ofdeposition (according to the pattern of charge upon or below thesubstrate surface) can be produced.

In FIGS. 5 to 7 is shown apparatus suitable for use with a liquid 30that incorporates species 31 that have a net positive electrical chargeand species 32 that have a net negative electrical charge. The liquid 30is brought to the vicinity of auxiliary electrode 28 and the rear faceof perforate plate or membrane 3 via an insulating supply duct 36. Theliquid 30 may, for example, be a liquid comprising an insulating carrierin which charged species 31 are mobile toner particles and chargedspecies 32 are mobile counter-ions. We use this example for theembodiments shown in FIGS. 5 to 7 to illustrate the case where it isdesirable to eject positively-charged droplets carrying toner particles,although other examples will be apparent to the person skilled in theart.

In FIG. 5 is shown an auxiliary electrode 28 in direct contact withliquid 30 and which is capable of receiving free electrical charge fromelectrical supply 11 at a potential V_(ch), which in this example istaken to be a positive potential with respect to the potential ofairspace 15 a short distance in front of plate or membrane 3. Perforateplate or membrane 3, which may be formed either of conducting or ofnon-conducting material, is vibrated in the direction shown at 6 causingcharged droplets 37 to be ejected into airspace 15 in the directionshown at 9. Replenishing supply of liquid 30 is provided by insulatingduct 36 in supply direction shown at 34 as liquid is lost from the plateor membrane perforations. As liquid 30 approaches the neighbourhood ofauxiliary electrode 28, species 32 are initially attracted towards andtoner particle species 31 are repelled away from that electrode.Consequently, in the region immediately adjacent auxiliary electrode 28liquid 30 acquires a net negative space charge from the raisedconcentration of counter-ions 32. Either by a low amount of counter-ionspecies 32 (and of toner particles 31), or by the supply of free chargeby auxiliary electrode 28 to counter-ion species 32, the space chargebuild-up in this region is limited and toner particles 31 experiencerepulsion from auxiliary electrode 28 towards perforate plate ormembrane 3. Therefore, ejected droplets 37 are formed with a netpositive charge and with a raised concentration of toner particles. Thisgeometry is also suitable for use with aqueous solutions, includingwater itself, in which case electrode 28 acts similarly to electrode ofthe charge donating assembly 14 of FIG. 1a.

In FIG. 6 is shown an alternative arrangement to that of FIG. 5 in whichperforate plate or membrane 3 is conducting and raised to potentialV_(ch), taken by way of example to be a negative potential with respectto the potential of airspace 15, by electrical supply 11 and in which itis electrically insulated from liquid 30 by a thin dielectric layer 38.In this example, auxiliary electrode 28 in contact with liquid 30 iscapable of receiving free electrical charge at ground potential.Positive space charge density arises in the region immediately behindperforate plate or membrane 3 due to the electrostatic attraction oftoner particles 31 towards perforate plate or membrane 3. Again, onejection of liquid as droplets from perforate plate or membrane 3,droplets 37 are formed with a net positive charge and with a raisedconcentration of toner particles. This geometry also operates withaqueous solutions and water, it is believed due to the effect ofelectrical fringing fields within the perforate regions of perforateplate or membrane 3.

FIG. 7 shows similar apparatus but in which auxiliary electrode 28 iselectrically insulated from the liquid so that it cannot supply freecharge to counter-ion species 32. In consequence, unless the totalamount of counter-ion species 32 or toner particles sufficientlylimited, the space charge adjacent to auxiliary electrode 28 andmembrane 3 may increase to such an extent that the resultant electricalfield within the liquid between auxiliary electrode 28 and perforateplate or membrane 3 prevents further migration of toner particles 32towards perforate plate or membrane 3. The inventors understand thatthis need not prevent ejection of charged, toner-rich droplets providedthe supply of liquid 30 along duct 36 and past perforate plate ormembrane 3 and auxiliary electrode 28 sweeps away at least part of thespace charge region of counter-ions adjacent auxiliary electrode 28. Ifa closed or recirculating liquid supply system is desired, however, a`downstream` electrode capable to supply free charge to the liquid asshown by dashed conductor 41 and electrode 42 in FIG. 7 allowsindefinite operation of the apparatus. In this case this embodiment isalso suitable for operation with aqueous solutions and water.

It is not required that droplet production is effected by action ofactuator 4 to vibrate perforate plate or membrane 3 or otherincorporating in use orifices contacted by liquid and circumscribingtheir menisci. Alternatively actuator 4 may induce vibrations (generallyultrasonic vibrations) within the liquid contacting the plate ormembrane 3, which may now advantageously be mechanically rigid. Anembodiment similar to that of FIG. 2 but in which actuator 4 inducessuch vibration within the liquid is shown in FIG. 8. A furtherembodiment in which an induction electrode 38 is employed is shown inFIG. 9.

Further embodiments similar to that of FIG. 5 and suitable for use withnon-conducting liquids carrying charged species components will beevident to the reader skilled in the art.

It is not required that the ejected droplets are ejected alreadycarrying an electrical charge. The charge can be imposed on dropletsfollowing their generation by perforate plate or membrane dropletgeneration apparatus of the types disclosed above. An example is shownin FIG. 10.

In FIG. 10 is shown droplet generating apparatus, which generally may beof any of the types disclosed above, used in conjunction with a corotronion source 50. The corotron ion source comprises a fine wire 51 raisedto a potential V_(ch) by electrical supply 11, at which potential theelectrical field in the air or other gas in the immediate vicinity ofwire 51 is sufficiently large to cause ionisation of the air (or othergas) to produce a stream of ions 52 that may be directed towards thedroplets 7. Impact of such ions with the droplets gives them a freeelectrical charge. Known refinements of the corotron that may be used toadvantage in this application include those as already described withreference, FIG. 1d, to the use of corotron charging of the substrate109, of a ground electrode (not shown) on the side of the wire 50furthest from droplets 7 and the provision of a so-called "gridelectrode", known in the electrographic arts, on the side of the wire 50nearest the droplets 7.

The best embodiment of the invention presently known to the inventorscomprises the general arrangement of FIG. 4 used in conjunction with thepreferred embodiment of droplet dispensation apparatus substantially asdescribed in co-pending application WO-A-95/15822 together with pressurecontrol of the liquid.

The detailed implementation used is as shown in FIG. 11. In oneexperiment with this arrangement tap water 100, whose conductivityexceeded 1 μS/m, was placed in a closed and insulated reservoir 90. Tothe base of the reservoir, a perforate membrane droplet device of thetype described in co-pending application WO-A-95/15822 was attached insuch a way as to form a direct electrical contact between the perforatemembrane 3 and the water, via a simple gravity feed.

Piezo-ceramic actuator 4 was electrically and mechanically coupled to ametallic substrate 70, in turn electrically and mechanically coupled toperforate membrane 3. No insulating layer 19 between the piezo-ceramicelement 4 and the substrate 70 was employed; instead the chargingpotential V_(ch) was applied by supply 11 directly to the substrate 70(and so to one electrode of the piezoelectric actuator 4 and theperforate membrane 3) via a center tap 81 on the secondary windings ofthe isolation transformer 80. This potential was varied between ±0 kVand ±1.8 kV. The primary of isolation transformer 80 was connected toalternating voltage supply 5, providing a sinusoidal voltage of 70 voltspeak to peak at the actuator 4 at frequency in the region of 280 kHz.

Perforate membrane 3 was 50 μm thick and formed of electroformed nickel;it included perforations 8 whose smallest diameter was 30 μm. Theseperforations were arranged on a triangular 200 μm pitch and were taperedperforations in such a way that the hole taper opens outwards into theair. This perforate membrane, with an overall diameter of 6 mm, wasbonded onto a 4 mm center diameter hole in a 300 μm thick stainlesssteel substrate 70 whose outer diameter was 20 mm. Onto the front faceof this assembly, a 200 μm thick piezoelectric ceramic annular actuator4, having continuous silver electrodes 4a and 4b fired onto andextending over its major faces, was electrically and mechanicallyattached. The outer diameter of annular actuator 4 was 14 mm and theinner diameter was 9 mm. It was of a type known as P51 from HoechstCeramtec.

A negative pressure, near to the pressure at which air entered theclosed reservoir 90 through perforations 8 was applied to the water 100within the reservoir. The induced vibration shown at 6 in the mesh,resulted in ejection of droplets 101 of water in direction 9 at anaverage flow-rate of 3.4 μl/s. The volumetric mean diameter of thedroplets was measured to be 10.1 μm using a commercially-availableMalvern Mastersizer S instrument.

An earthed induction electrode structure 71, having a central hole ofdiameter 8 mm was positioned a distance of 4 mm in the front of themembrane 3, through which the water droplets 101 were ejected. Thisgeometry was modelled using electrostatic modelling software to createat the surface of the perforate membrane a spread of 20% from the meanvalue electric field between induction electrode 71 and substrate 70 andmembrane 3.

Charge was found to be imparted to the droplets. The ratio of dropletcharge to droplet mass (Q/M) was measured by directing the dropletstream into a collection pot made of conducting material placed upon amass balance (not shown). An electrometer was connected between theconducting pot and electrical earth to measure the total charge ofcollected droplets, and the mass balance measured the total mass of thesame droplets. The charge to mass ratio Q/M was thereby determined andwas found to be approximately proportional to the potential V_(ch)provided by supply 11 with proportionality constant of 3×10⁻⁶ coulombsper kilogramme per volt.

This apparatus and closely-similar conditions were also employed usingan aqueous suspension of pigment particles at a solids volumeconcentration of 5%. When the produced droplet spray was brought in thenear proximity of the imagewise charged photoconductive substratepresented by a Hewlett-Packard® LaserJet 4 printer producing chargepatterns with high spatial resolution, the droplet stream depositedfaithfully upon the charged regions of the substrate and with little orno deposition on uncharged regions.

The best embodiments of the charging means used with the second aspectof the invention are standard forms of corotron used to deposit chargeupon a photoconductor surface, as generally described for example inSchaffert's book `Electrophotography` published by Focal Press.

The apparatus therefore advantageously allows delivery of chargeddroplets of aqueous toners in a manner suitable for imagewisedevelopment of charge patterns upon or below separate substrates toproduce high contrast image marks.

FIG. 12 shows a further example of a means for providing a pattern ofelectrical charge or potential (shown at 136) below the surface 131 of asubstrate 130 in a manner suitable for charged droplets 7 to depositupon that surface responsive to that charge pattern.

Substrate 130 in this case comprises a thin insulating layer ofmaterial, typically of thickness in the range 5 to 100 microns, with anupper face 131 exposed to droplets 7 having charge 7a (shown by way ofexample as a negative charge) produced by any of the embodiments ofcharged droplet production apparatus referred to above. In closeproximity to a lower face 132 of the substrate 130 is placed an assemblyof electrodes 133, partially shown in the figures as 133a, 133b, and133c. To each electrode 133a, 133b, 133c . . . is respectively appliedpotentials V_(a), V_(b), V_(c) (by way of example above groundpotential) shown at 136 by electrical supplies 134a, 134b, 134c . . .via conductors 135a, 135b, 135c . . . . Alternatively electricalsupplies 134a, 134b, 134c . . . may instead be operated to supply toelectrodes 133a, 133b and 133c fixed electrical charges q_(a), q_(b),q_(c).

The electrostatic field pattern produced by the potentials V_(a), V_(b),V_(c) . . . or charges q_(a), q_(b), q_(c) . . . located below thesurface 131 of the insulating substrate 130 (`below` being used in thesense of being on the face of substrate 130 more remote from thedroplets 7) extends above the upper surface 131 (`upper` being used inthe sense of being on the face of substrate 130 less remote from thedroplets 7) and charged droplets 7 deposit on to the surface 131responsively to those potentials or charges. By way of example only thesign shown at 7a of the charge of droplets 7 is shown to be opposite tothe sign of the potential or charge provided below the substrate surfaceshown at 136. In this way droplets 7 are attracted electrostatically todeposit preferentially upon the more highly charged or higher potential(as appropriate) of electrodes 133, as shown at 138a and 138c.

When the electrodes 133 are maintained at a constant electricalpotential, electrical charge in general flows into or out of thoseelectrodes as droplets 7 approach and deposit on to the surface 131.Typical values for such potential lies in the range 100 to 1000 volts.When, alternatively, the electrodes 133 are supplied by electricalsupplies 134a, 134b, 134c . . . with fixed amounts of charge q_(a),q_(b), q_(c) . . . the electrical potential of those electrodes changesas the droplets 7 approach and deposit on to the surface 131. (Theseeffects occur also where the electrical pattern is formed upon as wellas below the surface 131 of the substrate 130).

What is claimed is:
 1. A method of supplying material to a substrate,said method comprising:supplying liquid to a member having a surface,said surface having a plurality of features locating menisci of saidliquid at said surface; inducing mechanical vibrations within the liquidlocated by said features, thereby forming liquid droplets and causingsaid liquid droplets to be sprayed from said member; supplyingelectrical charge to the liquid before or as said droplets are sprayedfrom said member; and providing electrical charge or potential to thesubstrate, whereby said droplets are directed towards said substrate todeposit said material thereon.
 2. A method according to claim 1, whereinsaid spray is directed substantially parallel to said substrate.
 3. Amethod according to claim 2, wherein said electrical charge is suppliedconductively to the liquid through said member.
 4. A method according toclaim 1, wherein said liquid is supplied to said member at or belowambient pressure.
 5. A method according to claim 4, wherein saidelectrical charge is supplied to the liquid droplets after they aresprayed from said member.
 6. A method according to claim 5, furthercomprising inducing charge on said droplets by means of an inductionelectrode disposed on the side of said member adjacent to said surface.7. A method according to claim 1, wherein said electrical charge issupplied conductively to the liquid at one side of said member oppositeto said surface.
 8. A method according to claim 7, further comprisinginducing charge on said droplets by means of an induction electrodedisposed on the side of said member adjacent to said surface.
 9. Amethod according to claim 1, wherein electrical charge or potential issupplied to the surface of said substrate.
 10. A method according toclaim 1, wherein electrical charge or potential is supplied to thesubstrate on the side of said substrate remote from said member.
 11. Amethod according to claim 1, wherein electrical charge or potential issupplied to the substrate on the side of said substrate adjacent to saidmember.
 12. A method according to claim 1, wherein said providingelectrical charge or potential is supplied to the substrate by means ofa corotron ion source.
 13. A method according to claim 1, wherein thespacing between said droplets in a direction transverse to their path,their size and their speed is adapted to cause said droplets to entrainair during their flight, thereby to form a moving body of fluid.
 14. Amethod of providing an image on a substrate, the method comprisingforming said image from a material species carried by liquid droplets,wherein said material species is supplied to said substrate by a methodaccording to claim
 1. 15. A method according to claim 14, wherein saidspecies are disolved or suspended in said liquid.
 16. A method accordingto claim 15, wherein said liquid is formed, at least in part, of water.17. A method according to claim 15, wherein said species are chargedparticles or ions.
 18. Apparatus for depositing material supplied in theform of a liquid on to a substrate comprising:a member having a surfacefor receiving the liquid, a plurality of features at said surface forlocating thereat menisci of the liquid supplied to said member; anactuator for inducing mechanical vibrations within the liquid located bysaid features to cause liquid droplets to be formed and sprayed fromsaid member; liquid supply means for supplying liquid to the member;means for supplying electrical charge to the liquid before or as saiddroplets are sprayed from said member; and means for providingelectrical charge or potential to the substrate, whereby said dropletsare directed towards said substrate to deposit said material thereon.19. Apparatus according to claim 18, wherein said member comprises aplate.
 20. Apparatus according to claim 18, wherein said membercomprises a flexible membrane.
 21. Apparatus according to claim 18,wherein said surface is a planar surface.
 22. Apparatus according toclaim 18, wherein said features comprise orifices through said member.23. Apparatus according to claim 18, wherein said actuator comprises apiezoelectric transducer connected to said member to cause said memberto vibrate in use, thereby to vibrate said liquid to produce saiddroplets.
 24. Apparatus according to claim 18, wherein said actuatorcomprises a piezoelectric transducer disposed to vibrate said liquiddirectly to produce said droplets.
 25. Apparatus according to claim 18,wherein said liquid supply means supplies liquid at or below ambientpressure.
 26. Apparatus according to claim 18, wherein said means forsupplying electrical charge to the liquid comprises at least oneelectrode disposed to one side of said member opposite to said surfaceand arranged to contact said liquid supplied thereto whereby said chargeis applied conductively through the liquid.
 27. Apparatus according toclaim 26, further comprising an induction electrode disposed on the sideof said member adjacent to said surface to induce charge on saiddroplets.
 28. Apparatus according to claim 18, wherein said means forsupplying electrical charge to the liquid comprises at least oneelectrode disposed on said member whereby said charge is appliedconductively through the liquid.
 29. Apparatus according to claim 18,wherein said means for supplying electrical charge to the liquidcomprises means arranged to apply charge to said droplets after they aresprayed from said member.
 30. Apparatus according to claim 29, whereinsaid means for supplying electrical charge to the liquid comprises acharge emitting electrode disposed on the side of said member adjacentto said surface to induce charge on said droplets.
 31. Apparatusaccording to claim 29, wherein said means for supplying electricalcharge to the liquid comprises a corotron ion source.
 32. Apparatusaccording to claim 29, wherein said means for supplying electricalcharge to the liquid comprises an electrogasdynamic ion generator. 33.Apparatus according to claim 18, further comprising an auxiliaryelectrode disposed to one side of said member opposite to said surface.34. Apparatus according to claim 33, wherein said auxiliary electrodehas an insulated layer to insulate it from said liquid.
 35. Apparatusaccording to claim 18, wherein said means for providing electricalcharge or potential to the substrate is adapted to supply said charge orsaid potential on said substrate.
 36. Apparatus according to claim 18,wherein said means for providing electrical charge or potential to thesubstrate is adapted to supply said charge or said potential on the sideof said substrate remote from said member.
 37. Apparatus according toclaim 18, wherein said means for providing electrical charge orpotential to the substrate is adapted to supply said charge or saidpotential on the side of said substrate adjacent to said member. 38.Apparatus according to claim 18, wherein said means for providingelectrical charge or potential to the substrate includes a corotron ionsource.
 39. Apparatus according to claim 38, wherein said means forproviding electrical charge or potential to the substrate furtherincludes an illumination source for providing a pattern of illuminationon a substrate comprising a photoconductive material.
 40. Apparatusaccording to claim 18, wherein said means for providing electricalcharge or potential to the substrate includes a plurality of electrodesdisposed on a side of the substrate remote from the member, each of theelectrodes being supplied selectively in use with a respectiveelectrical voltage or charge.
 41. Apparatus according to claim 18,wherein said features are arranged in a two-dimensional array. 42.Apparatus according to claim 18, wherein said features are arranged in aline.
 43. Apparatus according to claim 18, wherein said means forproviding electrical charge or potential to the substrate is adapted toprovide said charge or potential in a pattern on said substrate or onthe side of said substrate remote from said member.
 44. An imagingapparatus for depositing a material species on a substrate to form animage thereon, said species being carried by liquid droplets, whereinthe imaging apparatus includes the apparatus according to claim
 18. 45.Apparatus for depositing material supplied in the form of a liquid on toa substrate comprising:a member comprising a flexible membrane having asurface for receiving the liquid, a plurality of features at saidsurface for locating thereat menisci of the liquid supplied to saidmember; liquid supply means for supplying liquid to the member; anactuator for inducing mechanical vibrations within the liquid located bysaid features to cause liquid droplets to be formed and sprayed fromsaid member; means for supplying electrical charge to the liquid beforeor as said droplets are sprayed from said member; and means forproviding electrical charge or potential to the substrate, whereby saiddroplets are directed towards said substrate to deposit said materialthereon.
 46. Apparatus for depositing material supplied in the form of aliquid on to a substrate comprising:a member comprising a flexiblemembrane having a surface for receiving the liquid, a plurality offeatures at said surface for locating thereat menisci of the liquidsupplied to said member; liquid supply means for supplying liquid to themember; a piezoelectric transducer connected to the member to cause saidmember to vibrate for inducing mechanical vibrations within the liquidlocated by said features to cause liquid droplets to be formed andsprayed from said member; means for supplying electrical charge to theliquid before or as said droplets are sprayed from said member; andmeans for providing electrical charge or potential to the substrate,whereby said droplets are directed towards said substrate to depositsaid material thereon.
 47. Apparatus for depositing material supplied inthe form of a liquid on to a substrate comprising:a member comprising aflexible membrane having a surface for receiving the liquid, a pluralityof features at said surface for locating thereat menisci of the liquidsupplied to said member; liquid supply means for supplying liquid to themember; a piezoelectric transducer for inducing mechanical vibrationswithin the liquid located by said features to cause liquid droplets tobe directly formed and sprayed from said member; means for supplyingelectrical charge to the liquid before or as said droplets are sprayedfrom said member; and means for providing electrical charge or potentialto the substrate, whereby said droplets are directed towards saidsubstrate to deposit said material thereon.
 48. Apparatus for depositingmaterial supplied in the form of a liquid on to a substrate comprising:amember comprising a flexible membrane having a surface for receiving theliquid, a plurality of features at said surface for locating thereatmenisci of the liquid supplied to said member; liquid supply means forsupplying liquid to the member; an actuator for inducing mechanicalvibrations within the liquid located by said features to cause liquiddroplets to be formed and sprayed from said member; means for supplyingelectrical charge to the liquid before or as said droplets are sprayedfrom said member; means for providing electrical charge or potential tothe substrate, whereby said droplets are directed towards said substrateto deposit said material thereon; and an auxiliary electrode disposed toone side of said member opposite to said surface.
 49. Apparatusaccording to claim 48 wherein said auxiliary electrode includes aninsulated layer to insulate said auxiliary electrode from said liquid.